Digital projection displays (also known as digital light projection or DLP systems) are in widespread use. In one form of such a system, a high-angle incident light beam from a light source is reflected from a TIR (total internal reflection) face of a TIR prism. (All angles discussed herein are measured from the normal or perpendicular line to a plane.) The reflected light is incident upon a spatial light modulator such as a digital micromirror device (DMD, sometimes called a digital mirror device). The spatial light modulator controllably reflects individual pixels of the incident light beam back into the TIR prism, through the TIR face at a sufficiently low angle that the light passes through the TIR face, and to a projection lens system to form the projected image. The light that is not to form a part of the projected image is reflected in another direction by the spatial light modulator. Digital light projection systems are of increasing interest because they are based upon digital storage of the image, and because they may be made highly compact.
As with all projectors, the image contrast achieved by the digital projection display is an important consideration. The image contrast is a subjective assessment of how strongly the image of interest (usually a darker area) differs from its background (usually a lighter area). If the image contrast is low, the image will not be clearly defined relative to the background, and will appear weak or diluted. If the image contrast is high, the image will stand out clearly against the background. The image contrast may be quantitatively expressed in terms of a “contrast ratio”, the ratio of the maximum to the minimum luminance in all or a selected portion of the field of view.
Most digital light projection systems have complex light paths and depend upon multiple light reflections from the digital micromirror device, the TIR prism, and a compact optical system. Consequently, there are several mechanisms by which the contrast ratio is degraded. Off-state light reaching the image, ghost images, and stray light all increase the amount of light that reaches areas of the projected image that ideally are dark (i.e., have no light). The result is that the image contrast and the contrast ratio are degraded, and the image is visually less pleasing to the observer.
There is a continuing need to improve image contrast and the contrast ratio of projectors generally, but particularly for digital projection displays. The approach adopted must be consistent with the otherwise-compact structure of the digital projection display. The present invention fulfills this need, and further provides related advantages.